The present invention relates to a belt member formed with a seam portion and incorporated in an image forming apparatus.
In an image forming apparatus, there is used a belt-shaped film such as a photosensitive film and an image fixing film. For example, Japanese Patent Publication No. 8-187773A discloses an image fixing film formed with an overlapped portion at which longitudinal ends of the film are overlapped. It is disclosed to form a seamed endless belt by bonding the overlapped portion (hereinafter, referred to as seam portion).
The seam portion is formed by a length shorter than a total length of the endless belt. When the seamed endless belt is supported between stretching members and driven to circulate, since the length of the seam portion is extremely shorter than a distance between the stretching members, there poses a problem that when the seamed endless belt is repeatedly used, a damage is caused such that the seam portion is exfoliated or the belt is cut.
FIGS. 31 and 32 are sectional views showing a constitution of a film 70 described in the above publication. In FIG. 31, numerals 71 and 72 designate longitudinal ends of a substrate, and a seam portion 72 is formed by adhering the overlapped ends. A thickness of the substrate at the seam portion becomes Wa to bring about a stepped difference.
There is a case in which the substrate having a stepped difference cannot be run smoothly or a case in which the substrate causes to produce damage. Therefore, as shown by FIG. 32, a flattened portion 73 is formed at the seam portion by pressing to flatten the stepped difference by applying heat and pressure.
According to the above constitution, only one sheet of the substrate is constituted except the seam portion and therefore, there is a case in which the strength is deficient when the film is used as a belt member of a photosensitive film, an image fixing film or the like in an image forming apparatus. Therefore, a problem of destructing the belt member is posed. Further, there poses a problem that the large stepped difference of the belt member in running impinges on other elements of the apparatus, so that the belt member is damaged.
Further, as shown by FIG. 32, since the seam portion is pressed to flatten to constitute the flattened portion, a density of the flattened portion becomes approximately twice as much as that of other portion. Therefore, when such a substrate is used as an image fixing film, transfer of temperature to a recording medium is deteriorated at the portion to thereby pose a problem that a failure in fixing is brought about.